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year], by a male that was successful in raising two broods, one
with each of two different females, both of which were
themselvespolyandrous. [bracketed part added 01 August 2001, for
clarity]
Females switched mates following either the success or failure
of a nest. No switching was observed during the actual care of a
clutchor brood. In 1983, a first-year banded female successfully
raised a brood of four with one male, then immediately moved to an
adjacent,unpaired male who had previously failed with an unbanded
female. Another, older banded female raised a brood of four with
one maleand then moved to an unmated male who had been constructing
a nest several territories away prior to the fledging of his
prospectivemate's first brood. He was then seen feeding at least
one of the two accompanying banded fledglings, while his new mate
proceeded tolay eggs in his nest. Such moves as these discounted
the interpretation that handling by humans may have affected the
behavior of thebirds, causing them to move to avoid
disturbance.
DISCUSSION
Within-season mate switching by females was sufficiently
frequent to suggest that males and females use different strategies
tomaximize reproductive success. Most passerine species have been
presumed to be strictly monogamous (Moller 1986), though
someexhibit varying degrees of polygamy (Fitch and Shugart 1984).
There have been no other reports of polygamous behavior among
theVireoninae. While an increasing number of articles have
described and at-tempted to explain advantages and causes of
polygamy (Orians1969; Leisler 1985), few examples of polyandry have
been documented among passerines. Most examples of polygamy
addresspolygyny in several small passerines (Moller 1986). Graul et
al. (1977) attempted to explain possible factors influencing
polygyny andpolyandry. Verner and Willson (1966) discussed the
influences that habitats might have on mating systems, and
predicted thatpolyandry should be a rare occurrence among
passerines.
Gowaty (1981), in describing mating strategies other than
polygyny, defined sequential polygamy as "mating and parenting
whichoccurs without significant overlap between successive mates,"
meaning that a female would contribute substantially to the raising
of a
first brood before attempting another brood with a new mate.
This strategy describes the sequence of events that I found at
GibraltarReservoir.
The apparent availability of unmated males in the study
population might be sufficient enticement to nearby females to
encouragemate switching, whether the females were successful or not
with their first mates. Indeed, Smith et al. (1982) suggested that
polygynywas a result of a similar, though opposite, sex bias in
Song Sparrows. Most such examples of polygyny, however, are of
primary andsecondary females simultaneously mating with one male
and brooding clutches presumably both fathered by the single male.
I foundno such simultaneity in the population I studied. In fact,
polygamous behavior appeared to be only one of several means that
femalesused to maximize their reproductive success. Many females
dwelt in large territories containing an abundant supply of nest
sites,allowing the pair to build numerous nests, even though all
failed. The availability of nearby, unpaired males did not seem to
distractmany repeatedly unsuccessful females from monogamy while,
simultaneously, other successful females switched mates
immediatelyafter completion of nesting duties in their first
territories, often traveling more than a mile to a new territory
containing a previouslyunpaired male.
The data on mate switching and between-year site fidelity
together show that males and females have developed different
strategiesregarding nest site selection and fidelity. Males
establish and maintain fixed territories, while females search for
one mate among manywho possesses a suitable nesting location.
Females view the entire area of habitat as one large potential
territory from which they mayselect one or more mates during their
lifetimes. Indeed, only 3 of 26 males retained the same mate from
one season to the next.
The strategy of the female, viewing the habitat area as one
large potential territory could result in greater genetic mixing.
The malebecomes more familiar with his territory and its resources,
so he is better able to provide the young with the necessary food
while theyare in the nest and to lead them to good foraging areas
once they have fledged. The female is free either to remain with
her young or tomove to another male with whom she can start a new
brood. Even though many successful broods were followed almost
immediatelyby another attempt with the same mate, the ability to
move to another male, whether or not the necessity arises, presents
the femalewith a greater array of opportunities for success than
she might have had if she remained strictly monogamous.
A clear understanding of the vireo's relationship with its
breeding habitats is important for management of this species. The
primaryimplication for proper management of its breeding habitats
is as follows: the males and females obtain a territory
differently, with the
males being tied much more closely to a certain plot of habitat.
Managers should be aware that actions that disturb or alter the
vireo'shabitats might not affect the more tenacious male but could
be sufficient to drive females from the disturbed area. The male
LeastBell's Vireos that are scattered on isolated territories
throughout southern California may be defending territories that
are inadequate tomeet a female's needs. The presence of a singing
male does not establish the presence of a female or that the
habitat is suitable for abreeding pair.
Mistakes of past management of other endangered passerines
should be used as lessons in methods to be avoided or altered by
managersof the Least Bell's Vireo. Understanding of these errors
should be combined with factual data and the proper understanding
of these data.With such a synthesis, we can better ensure that the
Least Bell's Vireo and its breeding habitats will recover their
place as viablemembers of our natural heritage.
ACKNOWLEDGMENTS
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S. Laymon ran statistical tests and helped edit the paper. M.
Freel continued Forest Service support begun by P. Schempf. The
U.S.Fish and Wildlife Service, through P. Sorensen and S. Wilbur,
encouraged and supported research. A. Craig of California
Departmentof Fish and Game helped find funds to continue the
research. [Santa Barbara and several Southern California chapters
of the NationalAudubon Society provided funding for supplies] V.
Gray conducted invaluable vegetational data collection and
analysis. P. Collins andD. Schroeder offered criticism and advice
on methods and analysis of field work. P. Collins and V. Gray
provided editorial assistancewith manuscripts.
LITERATURE CITED
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bellii Audubon. Univ. Kan. Publ. Mus. Nat. Hist. 12(3):241-296.
Fitch, M.A., and G.W. Shugart. 1984. Requirements for a mixed
reproductive strategy in avian species. Am. Nat. 124:116-126.
Goldwasser, S., D. Gaines, and Wilbur, S.R. 1980. The least
Bell's vireo in California: a de facto endangered species. Am.
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Gowaty, P.A. 1981. An extension of the Orians-Verner-Willson
model to account for mating systems besides polygyny.
Am.Nat.118:851-859.
Graul, W.D., Derrickson, S.R., and Mock, D.W. 1977. The
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